Conventional control technology in the steelmaking industry relies on static computer models, e.g., feed-forward heat and material balance computer models. The composition and weight of starting materials (e.g., pig iron, scrap steel, oxygen, fluxes, etc.) are input to the model, and oxygen flow rate and blowing time are calculated by the model. Due to the unknown composition of the scrap steel, significant inaccuracies occur in achieving the desired endpoint concentration of carbon and melt temperature in the final steel product. However, no practical real-time method is conventionally available for monitoring process characteristics of the steelmaking process, and therefore no dynamic control of a process can be performed.
The unavailability of real-time process monitoring and dynamic process control causes significant safety and efficiency problems for current steelmaking processes, such as the basic oxygen process. This process combusts oxygen with carbon contained in molten metal to transform the molten metal to steel. Safe and efficient transformation requires that process variables be controlled to maintain certain process characteristics (e.g., temperature and concentrations of CO, CO.sub.2, and H.sub.2 O) at preferred or optimum values. Since there presently is no on-line, real-time method to measure or monitor these process characteristics, dynamic adjustments cannot be made to ensure maximum safety and efficiency.
More particularly, current commercial methods for producing steel using the basic oxygen process involve transforming starting materials containing a relatively high carbon content (up to about 4 wt %) by blowing high velocity oxygen into the starting materials in a batch process called a "heat." The oxygen combusts with carbon contained in the starting materials to decrease the carbon content, resulting in steel containing carbon at levels of 0.03 to 0.6 wt %, depending on the desired alloy. The starting materials include molten metal and flux (collectively referred to as the "bath"), and during the steelmaking process they form both melt and slag phases in a furnace.
The process is carried out in a basic oxygen furnace (BOF), which includes a large vessel with a refractory lining for containing the bath, an oxygen lance for blowing oxygen into the bath, and exhaust ducts for removing gases produced by the steelmaking process. These gases are collectively referred to as the off-gas. One of the principal gases produced by the process is carbon monoxide (CO)--the primary reaction product of oxygen and carbon. A variety of schemes exist to further react this CO with oxygen prior to its exit from the furnace, to form carbon dioxide (CO.sub.2). This post-combustion reaction of CO and oxygen is highly exothermic, and therefore it releases heat to both the slag and the melt phases in the furnace. This additional heat accelerates the steel conversion process.
A significant problem with commercial steelmaking is making efficient use of the CO, i.e., efficiently controlling post-combustion of CO, which requires proper control of the oxygen flow rate. If insufficient oxygen is injected, then the maximum effective use of CO is not achieved. On the other hand, if too much oxygen is injected, then the process is not cost effective because oxygen is wasted and the off-gas becomes too hot, deleteriously affecting the refractory lining and the exhaust duct walls. Conventionally, post-combustion of CO is monitored in a time-averaged fashion using commercially-available mass spectrometry (MS) or non-dispersive infrared absorption (NDIR) methods. These methods require that a sample of the off-gas be extracted, cooled, and analyzed, and therefore there is a significant time delay in acquiring data. Some indication of post-combustion gas concentrations may also be derived by monitoring wall and cooling water temperatures in the exhaust ducting using standard thermocouple technology. However, this technique is severely limited in sensitivity, accuracy, and response time. This lack of on-line, real-time measurement of CO and CO.sub.2 concentrations in the off-gas prevents efficient control of the oxygen flow rate in conventional steelmaking processes to ensure optimal post-combustion of CO.
Another major problem with current commercial steelmaking methods is the lack of an on-line method to provide continuous, real-time data on the carbon content of the metal. In many commercial steel mills, the carbon endpoint concentration is determined by stopping the process at the predicted endpoint, extracting a sample of the molten steel, and performing an offline chemical analysis. Another technique involves using "sensor lance" technology, which requires the lowering of a water-cooled lance equipped with an expendable sensor into the furnace. The expendable sensor is immersed in the liquid steel near the predicted endpoint of oxygen blowing, a metal sample is extracted, and the cooling curve of the sample is measured. This cooling curve can then be related to the carbon concentration of the steel. Both of these methods yield only a single data point per heat rather than continuous data.
Other methods for carbon endpoint verification use MS or NDIR absorption methods to determine CO and CO.sub.2 concentrations in the exhaust gas after it has been cooled and particulates have been removed. These methods require the use of an off-gas treatment system to treat the gas, and such a system requires extensive maintenance.
As a result of the lack of continuous data, steelmakers sometimes resort to a technique in which a heat is "blown flat" to ensure that the molten metal has been adequately decarburized. The "blown flat" technique uses excess oxygen to reduce the carbon concentration in the melt down to the lower limit of the desired range, i.e., 0.03 wt %. After the vessel is tapped and the steel is transferred to a ladle, the carbon concentration is adjusted back up to the desired level by adding material in the ladle.
However, this process is inefficient for several reasons. First, excess oxygen is used to ensure the complete oxidation of carbon in the melt, necessitating an additional expense. Second, the use of excess oxygen causes iron in the molten bath to begin to oxidize when the dissolved carbon has been reduced to very low concentrations. This oxidation results in a loss of iron, which should form a portion of the commercial product, to iron oxide that ends up in the slag phase. Finally, the process of blowing flat requires the expenditure of unnecessary additional processing time, thereby reducing throughput in the industrial process and consequently increasing costs and lowering profits.
Another significant problem with current steelmaking methods is the creation of a highly reactive, foaming slag layer when certain combinations of hot metal chemistry and flux additions are used, which causes ejection of large amounts of liquid slag from the BOF vessel during oxygen blowing (referred to as "slopping"). This slopping causes undesirable rapid slag build-up on the vessel mouth and exhaust hood surfaces, and increased skull build-up on the lance. Slopping can be controlled by adjusting the lance parameters (e.g., the lance height above the bath and the oxygen flow rate), but this is difficult to achieve automatically, since current steelmaking techniques cannot detect the amount of liquid slag being ejected from the furnace.
Yet another significant difficulty with current steelmaking methods results from the presence of a large amount of water in the furnace. A large amount of water in the furnace causes rapid formation of molecular hydrogen at the melt surface, thereby creating a major safety hazard due to the risk of an explosion. The presence of excess water may be caused by unwanted leaks of cooling water from a water-cooled oxygen lance or water-cooled off-gas exhaust ducts. Conventionally, the potential for hydrogen formation in the steel conversion furnace is determined by monitoring input and output cooling water flows through the oxygen lance and an exhaust hood system. Large discrepancies or sudden changes in flow rates could indicate dangerous leaks into the furnace vessel. However, while water flow monitoring may indicate the source of water that may cause dangerous levels of hydrogen, this method does not indicate the levels of hydrogen actually present, and therefore this method fails to offer adequate safeguards against explosions.
In an effort to address the above-noted limitations of conventional monitoring techniques, research has been conducted on methods for sampling gas near the furnace mouth using water-cooled extractive probes. The extracted gas is then analyzed, either with Fourier transform infrared (FTIR) spectroscopy or mass spectrometry. The FTIR method provides a relatively real-time response for measuring gas-phase concentration, as compared with the methods described above. However, an extractive probe has a limited life due to its location above the furnace mouth. Further, the off-gas temperature is measured in this technique by a thermocouple located in the extraction probe, and such thermocouples have slow response times.
Accordingly, an improved monitoring method is needed that can provide real-time, continuous data about off-gas characteristics using a reliable, non-intrusive method.